[Proj] Re: Can I set the unit of Mercator Porjection? I'm trying to overlay shp file to google map.

[baijianhua]

Frank Warmerdam <warmerdam <at> pobox.com> writes:

> 
> baijianhua wrote:
>  > I'm trying to overlay shape files(world map) to google map, I found that 
> google
> > map is using this PROJCS.
> > 
> > 54004,PROJCS["World_Mercator",GEOGCS["GCS_WGS_1984",DATUM
["WGS_1984",SPHEROID
> > ["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT
> > ["Degree",0.017453292519943295]],PROJECTION["Mercator_1SP"],PARAMETER
> > ["False_Easting",0],PARAMETER["False_Northing",0],PARAMETER
> > ["Central_Meridian",0],PARAMETER["Standard_Parallel_1",0]
> > 
> > ,UNIT["Meter",1]
> > 
> > ,AUTHORITY["EPSG","54004"]]
> > 
> > 
> > I can get the same shape of google map(in world range, the top most level 
of 
> > google map), but one mismatch is the unit, the unit google map used is 
Decimal 
> > Degree, while Mercator Projection using is Meter, when I change this to 
> > 
> > 54005,PROJCS["World_MercatorDD",GEOGCS["GCS_WGS_1984",DATUM
["WGS_1984",SPHEROID
> > ["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT
> > ["Degree",0.017453292519943295]],PROJECTION["Mercator_1SP"],PARAMETER
> > ["False_Easting",0],PARAMETER["False_Northing",0],PARAMETER
> > ["Central_Meridian",0],PARAMETER["Standard_Parallel_1",0]
> > 
> > ,UNIT["Degree",0.0174532925199433]
> > 
> > ,AUTHORITY["EPSG","54005"]]
> > 
> > Still I can not get the same coordinate value as google map, can anybody 
help? 
> > can I using the same unit with google map in Mercator projection result
> 
> Baijianhua,
> 
> It is not generally meaningful to use degrees as a linear unit for a
> projected coordinate system in PROJ, OGR or simple features well known text
> coordinate system descriptions.
> 
> Perhaps it is a property of mercator that a degree is linear in the
> latitude direction (and equal number of mercator meters at any latitude),
> and that it is the same distance in the longitude direction at any
> latitude.   If so, feel free to put in a UNIT["degree",111111.0] or whatever 
> the correct number of mercator meters for this pseudo-linear
> unit is.
> 
> PS. I know I should know more about the properties of Mercator.  So shoot
> me!
> 
> Best regards,



Thanks for response. I think you are right that it doesn't make sense to use 
degrees as a linear unit for a projected coordinate system in PROJ, but I'm 
wondering why google map can use Decimal Degree as the coordinate for markers, 
do you think they are doing some kind of internal coordinate system 
transformation? And, I think it's also very common that we render 2 dimension 
map using decimal degree as unit. So I'm a little confusing what we are 
actually doing when we talking about decimal degree in 2 dimension map. Forgive 
me that maybe I'm asking stupid questions, I'm not very famirliar in this field.